MAP kinase kinase 1 (Mek1), also known as mitogen activated protein kinase kinase, MAP kinase kinase, ERK kinase 1 or microtubule-associated protein 2 kinase, is a 43 kDa dual-specificity protein kinase that phosphorylates threonine and tyrosine residues on MAP kinases such as Erk 1 and 2. Mek1 and Mek2 are involved in a diverse array of cellular processes such as stress-activated response, apoptosis, cytokine-induced cell proliferation, and DNA recombination during meiosis. Mek1 and Mek2 are activated by phosphorylation on serine 218 and 222 by serine-threonine kinase, Raf-1, which is part of the p21ras signal transduction pathway.